Incorrect inferred type for attribute with two partial types

[JukkaL]

Unexpected type is inferred for attribute x in this example:

class C:
    def __init__(self) -> None:
        if int():
            self.x = None
            return
        self.x = []
        self.x.append("x")

reveal_type(C().x)  # "list[str]" -- but should be "list[str] | None"

The inferred type should be list[str] | None, or alternatively mypy could require a type annotation for x, if this is too tricky to infer.

This should be tested with and without --allow-redefinition-new, since it may make a difference.